1. Field of the Invention
This invention relates to an improved rotational energy absorber and in particular to an improved rotational energy absorber for use in a fall arrest system.
2. State of the Art
Fall arrest systems are used to prevent personnel working at a height from suffering injury or death due to falls. Fall arrest systems are also often referred to as height safety systems or fall prevention systems.
A common type of rotational energy absorber used in fall arrest systems is a friction brake type energy absorber. This generally comprises two circular, or annular, friction pads, held in face to face contact with a preset load between them. When a sufficiently large couple is applied between the two pads, relative rotation of the two pads will take place and the friction between the pads will absorb energy. In a fall arrest system the rotational energy absorber is used to absorb the kinetic energy of a falling user, so slowing and then arresting their fall.
There are a number of problems with such frictional rotational energy absorbers. Firstly, there is the fundamental problem that the applied couple required to start relative rotation of the two pads is larger than the couple required to continue rotation. This is a particular problem in a fall arrest system because there is a maximum safe load which a falling person can be subjected to while their fall is being arrested. The fall arrest system must be arranged so that the load applied to a falling user by the couple required to start relative rotation of the friction pads is not larger than the safe limit, so it follows that the load applied to the falling user by the smaller couple required to continue relative rotation of the friction pads will be less than the safe limit. As a result the rate at which the energy of the fall is absorbed is less than could be achieved if the load applied to the falling user was constant at the safe limit, so that the falling user will fall further before their fall is arrested and the user brought to a stop. The extra distance fallen increases the risk that the falling user will be injured by impact with obstacles, or the ground. Further, the extra distance fallen increases the total amount of energy which must be absorbed, requiring an increase in the energy absorbing capacity of the energy absorber, and thus an undesirable increase in size, weight and cost.
Further, the degree of friction between the two pads is dependent upon the contact load acting between the two pads. As a result, in order to set the couple required to start and continue relative rotation of the pads to a desired value, such frictional energy absorbers require precise setting of the contact load. Further, the mechanism used to set the contact load, generally some form of spring, suffers from the problem of the applied load changing over time due to environmental effects.
Finally, the couples required to start and continue the relative rotation of the pads is highly sensitive to the surface properties of the pads. These properties are subject to the problem of the required couple changing over time due to environmental effects.